The present invention relates to a system for the actuation and positioning of a valve, and more particularly to a system that incorporates pneumatic and hydraulic features such that the pneumatic and hydraulic sections combine to control movement of a valve actuator, while the hydraulic section controls the power in a feedback-based control system. This xe2x80x9cair over hydraulicsxe2x80x9d (alternately referred to as pneumatic-hydraulic) actuator system enhances valve performance by providing a high function and low maintenance actuator that has significant application in explosion- and fire-susceptible control valve market segments, such as the liquid natural gas (LNG) production market.
The use of pneumatically and hydraulically controlled valves in process and fluid-handling operations is well known. In many large-scale applications, an electro-hydraulic valve actuation system is employed, where a centralized power unit is typically spaced apart from one or more valves through a network of high-pressure hydraulic lines. Based on actuator stem position in relation to a particular process need, a differential transformer provides a feedback signal to direct the opening and closing of one or more of the valves. The presence of a device based on electronic circuitry, while benign in many applications, can be disastrous when placed around inflammable process fluids, as a spurious electrical signal can be enough to ignite the fluid or fluid vapors. All-pneumatic systems can alleviate the concerns over electric sources placed in proximity to flammable fluids; however, these systems are often deficient in that they cannot provide quick response times and high load capacity. In addition, the compressible nature of the fluid (typically air) in a system built predominantly on pneumatics can lead to instability problems. All-hydraulic systems, while capable of providing rapid, stable response, also suffer from various limitations, including system complexity, as well as leakage, maintenance and safety features associated with high pressure fluid lines.
In a typical pneumatic-hydraulic actuation system, there are two discrete circuit portions: a low pressure pneumatic circuit, and a high pressure hydraulic circuit. The pneumatic portion provides power to pumps and valves used to transport hydraulic fluid in the hydraulic portion, which sends hydraulic fluid to an actuator to reposition a working fluid control valve. Unfortunately, traditional pneumatic-hydraulic systems either employ single or limited discrete operational modes, thereby limiting their valve responsiveness and consequent system accuracy. In addition, valves in traditional pneumatic-hydraulic systems often rely on additional electronic circuitry or components to effect valve actuation. The presence of such electronic componentry and the signals they carry can, in a flammable environment, act as an ignition source. Furthermore, maintaining fluid pressures in an energized, high pressure state is not cost effective, as the pump cycles frequently or continuously to overcome system pressure losses due to leakage, thereby increasing operating expense. By way of example, compressor recycle valves used in LNG systems require, in addition to explosion proof operation, more continuous (and hence responsive) flow of fluid through the working fluid control valve. In such applications, the use of traditional pneumatic-hydraulic systems can result in substandard performance.
Accordingly, there exists a need for a valve actuation system that can offer the simplicity and safety of pneumatic-based systems and the responsiveness and load capacity of hydraulic-based systems, in combination with safety and operability-enhancing features to enable efficient, reliable and inexpensive valve actuation, especially in safety-critical applications.
This need is met by the present invention wherein a hybrid valve actuation apparatus combining the best features of pneumatic- and hydraulic-based systems is utilized. According to one aspect of the present invention, a valve actuation system is disclosed. This valve actuation system includes a valve stem position indicator coupled to a pneumatic positioner, a high pressure circuit for circulating hydraulic fluid, a low pressure circuit for circulating pneumatic fluid, and a saturated fluid feedback circuit. Components within the high pressure circuit include a hydraulic fluid reservoir, a pump, an accumulator to store pressurized hydraulic fluid being discharged from the pump, a hydraulic actuator to manipulate the position of a valve in a main fluid transport system (also referred to as a working fluid valve), a servo valve with both pneumatic and hydraulic fluid paths such that the flow of hydraulic fluid through the hydraulic fluid path is proportional to the flow of pneumatic fluid through the pneumatic fluid path, and a trip manifold to facilitate retraction of the actuator upon attainment of a preset condition. Components within the low pressure circuit include a pneumatic fluid supply, a pneumatic positioner to respond to changes in valve actuator position received from the valve stem position indicator and an external command signal, and a solenoid operated valve to selectively permit pneumatic fluid to flow to the trip manifold. The saturated fluid feedback circuit includes a shutdown manifold and a binary valve. The binary valve can intermittently cut off hydraulic fluid flow to the servo valve upon attainment of a preset saturation condition. Optionally, the trip manifold further comprises an interconnected valve arrangement alternately comprising a first configuration such that normal servo valve operation is enabled, and a second configuration wherein the interconnected valve arrangement is such that the trip manifold disables the servo valve. This second configuration is mutually exclusive to that of the first configuration in that any flow control valves internal to the trip manifold that are open to permit flow in the first configuration are likewise closed to cut off flow in the second configuration, as any internal flow control valves closed during the first are opened during the second. In addition, the trip manifold is in fluid communication with the accumulator, hydraulic actuator, shutdown manifold and solenoid operated valve, as well as with the reservoir and binary valve.
The use of the servo valve, with its proportional response, provides more controllable articulation of a working fluid valve than is achievable with stepwise actuator positioning. A further advantage of using a servo valve according to the present invention as compared to a traditional electro-hydraulic actuation system is that the only energy source for actuator operation comes from readily available pneumatic fluid supply, preferably in the form of instrument air supply which, due to its lack of electronic componentry, is inherently safe in flammable environments. In addition, the incorporation of a saturated fluid feedback circuit is important in that an intrinsic part of the operation of any pump is its duty cycle, or duty factor, which is generally expressed as a ratio between the time a device is operating and the total time for an intermittently operating device. Thus, a duty cycle of one means the pump is operating continuously. Since the pump is a significant power draw, and that by continual operation its parts are wearing out quicker, it stands to reason that lowering its duty cycle without concomitant reduction in system operation would be beneficial from both a cost and component life perspective.
According to another embodiment of the invention, a fluid handling system with at least one working fluid valve being manipulated with an air over hydraulic actuation device is disclosed. In addition to the valve actuation component configuration of the previous embodiment, the fluid handling system comprises a network of piping configured to transport a fluid with at least one working fluid valve to regulate the flow of the fluid through at least a portion of the piping. The air over hydraulic approach of the present embodiment is particularly useful in fluid handling systems that transport inflammable materials, such as LNG. In one application of the present embodiment, for example, the working fluid valve being manipulated could be a compressor recycle valve where high thrust, fast speed and low maintenance in an explosion-proof environment are required.
According to another embodiment of the invention, a method of using a valve with an air over hydraulic actuation device is disclosed. This method utilizes the valve actuation structure discussed in the previous embodiments, and includes arranging a working fluid valve coupled to an actuator and a valve stem to be disposed within a fluid handling system, arranging a valve actuation system to use both hydraulic and pneumatic fluids to control a working fluid flowing through the working fluid valve, sensing a valve stem position with a valve stem position indicator, relaying the sensed valve stem position to the pneumatic positioner through a common linkage between the valve stem position indicator and the pneumatic positioner, generating a difference signal by comparing the sensed valve stem position to a command signal, sending the difference signal to the servo valve to vary the flow of pneumatic fluid through the pneumatic fluid flow path in proportion to the magnitude of the difference signal, adjusting a flow of hydraulic fluid through the hydraulic fluid flow path in proportion to flow changes through the pneumatic fluid flow path, moving the actuator in response to changes in pressure caused by flow of the hydraulic fluid flowing through the hydraulic fluid flow path, positioning the trip manifold to facilitate retraction of the actuator upon attainment of a preset condition, and operating a saturated fluid feedback circuit to intermittently stop the pump, thereby effecting a reduction in the duty cycle. By this method, proportional working fluid valve actuation is effected. In addition, the automated intermittent shutting down of the pump has the effect of reducing both operating costs (via shorter periods of electricity consumption to operate the pump) and maintenance costs (due to lesser wear and tear on the pump).